Exhaust gases produced by burning fuels using air as the source of oxidant typically contain small but significant amounts of various sulfur oxides (usually SO.sub.2 or SO.sub.3). SO.sub.2 is a participant in the photochemical reaction creating modern "smog" and, therefore, is undesirable.
There are a number of ways in which the SO.sub.2 may be removed or treated; however, each such process strongly benefits from use of an accurate monitor for detecting low levels of SO.sub.2. However, there are few low level SO.sub.2 sensors available which are practically suitable for inclusion in closed-loop controllers. Many of the prior measurement devices lack sensitivity. An ability to measure SO.sub.2 content in combustion gases at levels below 150 ppm with some accuracy is desireable.
One SO.sub.2 measuring procedure involves the use of an infrared beam, detector, and a comparator chamber. In U.S. Pat. No. 4,647,777 to Meyer a beam of infrared light is passed through a gas sample and into a selective infrared detector. The beam is split and one portion passes through a chamber containing a fluid which absorbs the spectral wavelengths of the selected gas. The two beams are compared and the difference between the two beams gives an indication of the amount of selected gas in the sample.
U.S. Pat. No. 4,836,012 to Doty et al. shows a semiconductor device made up of a photovoltic cell which, upon exposure to light, develops a voltage or current which varies as a function of the type of gas sorbed. The device requires a "thin light-transmitting gas-absorbing metal Schottkey layer having electrical properties which vary with the type of gas sorbed". Detection of CO, hydrocarbon, water vapor, etc. is suggested; detection of SO.sub.2 is not.
The U.S. Pat. No. 4,778,764 to Fine describes a device and a process in which a sample is injected with a solvent into a chromatographic column to separate the various materials present in the sample. The output of the column is then burned in the presence of a variety of detectors for one or more of NO.sub.x, SO.sub.2, CO.sub.2, and halogens.
None of the above disclosures suggest a process or an apparatus in which a catalytic sensor element is used to detect the presence of a gaseous component.
The concept of using the temperature rise of a gas as it reacts in passing through a catalyst bed as an indicator of the content of a component of that gaseous mixture has been shown. For instance, in U.S. Pat. No. 2,751,281 to Cohen, a method is taught for measuring low concentrations of gas impurities (such as oxygen) in the range of 0.0001% to 0.001%. A thermocouple is placed such that a cold reference junction is on the upstream side of a bed of catalyst and the hot junction is placed on the downstream side of that bed. As the gas flows across the catalyst, the temperature of the gas rises, is measured, and the content of the incoming gas calculated.
U.S. Pat. No. 3,488,155 to Ayers shows a similar process in which the temperature on each side of a hydrogenation catalyst bed is measured during the flow of a gas containing hydrogen. The temperature difference is related to the hydrogen content of the incoming gas stream.
The U.S. Pat. No. 3,537,823 to Ines suggests a process for measuring the quantity of "smog forming hydrocarbons in a gas sample" by measuring the temperature rise in an oxidation catalyst bed. Moreover, a related process is found in U.S. Pat. No. 3,547,587 also to Ines.
U.S. Pat. No. 3,607,084 to Mackey et al. teaches a process for the measurement of a combustible gas content by locating a pair of wires in a small chamber containing a volume of gas with combustibles therein. One wire is coated with a catalytic mixture of a metal oxide and a powdered metal of the platinum group and the other is apparently uncoated. Electrical power supplies heat to both wires. The difference in resistance caused by the change in temperature of the wire coated with the catalytic mixture provides an indicator of the amount of combustibles in that gas chamber.
U.S. Pat. No. 4,170,455 to Henrie also suggests a method for the monitoring of the hydrogen or oxygen content of a gas stream by measuring the temperature upstream and downstream of an oxidation catalyst.
U.S. Pat. No. 4,343,768 to Kimura shows a gas detector formed using semiconductor technology. The detector uses dual heating elements over a channel adapted for gas flow. One of the heating elements is coated with a "catalytic or gas responsive film" which may be platinum or palladium. The increase in the temperature of the catalytic film is detected in terms of the variation in electrical resistance in the content of the gas stream calculated.
Finally, U.S. Pat. No. 4,355,056 to Dalla Betta et al. suggests a differential thermocouple combustible sensor in which one junction of the thermocouple is catalytically coated and the other junction is not. The gas stream contains such gases as carbon monoxide and hydrogen and is said to be "insensitive to contaminants such as SO.sub.2 and NO".
None of these disclosures suggests a process for catalytically oxidizing all of the oxidizable materials except SO.sub.2 in the gas stream to be measured and then oxidizing the SO.sub.2 to SO.sub.3 on a separate catalytic element.
This invention is a sensor assembly having a catalytic oxidation preconverter and a functionally specific configuration of a catalytic sensor element having an integral thermally isolated temperature measuring device, e.g., a thermistor or RTD. Another portion of the sensor may be a temperature reference element.
This sensor configuration, particularly in conjunction with the inventive process, permits fast resolution of the SO.sub.2 content of the gas passing by the sensor to a high degree of accuracy and is insensitive to interference from other gas components in the measured stream.